This invention relates to semiconductor devices, and more particularly to a method of forming precision resistors compatible with a self-aligned silicided CMOS process for making integrated circuits.
In high-performance, high-speed integrated circuit devices the signal paths between integrated circuit chips are effectively transmission lines. The impedances of the input and output circuits at the chip are different from the impedances of the signal paths, which causes reflections and degrade the signal. On-chip resistors to match the resistance on the chip with that of the signal path are the most effective way of reducing these signal-degrading reflections. For proper matching, the resistors preferably have selected values in the 10-50 ohm range with tolerances of +/-10% (3 sigma). In MOS integrated circuits made by commercial processes, these resistance and tolerance requirements are best met by using resistors formed by N+ diffusions.
Resistors are formed in integrated circuit devices by making use of a diffused region of the proper length, width and impurity concentration to give the necessary resistance. In prior devices, the diffused regions creating the resistors are formed at the same time as source/drain regions in MOS integrated circuits. In contemporary CMOS processes using self-aligned silicided source/drain regions, however, the resistors cannot be created by previous methods without introducing unduly burdensome additional process steps, and these may have required critical alignments.
Various other methods for making resistors elements in integrated circuits have been proposed. For example, resistors may be formed in first-level or second-level polysilicon layers as set forth in U.S. Pat. Nos. 4,110,776, 4,209,716, 4,291,328 or 4,416,049. Alternatively, resistor elements may be formed by implanted regions buried under field oxide as set forth in U.S. Pat. No. 4,212,083. These prior methods are not suitable for the present purposes, however, due to the additional process steps needed, or deficiencies in the values or tolerances of the resistors, or their temperature coefficient of resistance, or other characteristics.